Monochorionic, monoamniotic twins gestations (Monoamniotic twins)

1. What every clinician should know

Monochorionic, monoamniotic (MCMA) twins are the product of a single fertilized ovum, resulting in genetically identical fetuses. If the ovum divides between 8 and 13 days after fertilization, MCMA twins are the result. MCMA twins are characterized by a common placental mass (monochorionicity) and a shared amniotic sac (monoamnionicity).

Despite increases in twinning rates across the developed world, MCMA twins remain a rarity, representing approximately 1% of all monozygous twins. The overall incidence of MCMA twins ranges widely across published reports, from 1:1,650-1:93,734 pregnancies. It is not known if increased use of assisted reproductive technologies has influenced the incidence of MCMA twinning.

2. Diagnosis and differential diagnosis

A. Establishing the diagnosis

When managing a twin pregnancy, it is of paramount importance to sonographically establish chorionicity at an early gestational age. Prior to 10 weeks gestational age, diagnosis of a monochorionic twin pregnancy can be established through the identification of a single gestational sac containing two embryos. The absence of a dividing membrane is required for the diagnosis of a MCMA pair; however, the thin inter-twin membrane of a monochorionic, diamniotic twin (MCDA) pregnancy may not be easily visualized at these earlier gestational ages. Yolk sac number may be used to support a diagnosis of MCMA twins. Generally, but not always, a single yolk sac is observed with MCMA twins and two yolk sacs indicate a MCDA pair.

At later gestational ages, MCMA twins may be diagnosed following detection of a shared placental mass with absent dividing membrane. Fetuses are also expected to demonstrate the same sex given monochorionic placentation. Most, but not all, MCMA twins display evidence of cord entanglement, which is diagnostic of monoamnionicity.

B. Differential diagnosis

The differential diagnosis for a MCMA twin pregnancy includes a MCDA pregnancy where there is a failure to visualize a dividing membrane. Complications that lead to a "stuck" twin presentation such as premature rupture of membranes, certain fetal genitourinary malformations, selective fetal growth retriction and twin-twin transfusion syndrome can lead to a false diagnosis of monoamnionicity.

In a pregnancy with history of a prior invasive diagnostic or therapeutic procedure, a large iatrogenic septostomy may also result in the sonographic appearance suggestive of monoamnionicity. However, in these cases a ribbon or strand-like remnant of the disrupted dividing membrane is usually sonographically apparent.

3. Management

While first trimester aneuploidy screening may be pursued with twin pregnancies, data are limited and suggest reduced detection rates for twin compared to singleton gestations. As with MCDA twins, aneuploidy screening using nuchal translucency (NT) measurements for MCMA twins is also potentially confounded by the association of discordant inter-twin NT measurements with TTTS risk and cardiovascular malformations, which are more likely in MCMA twins.

Patients presenting with twin pregnancies should be offered the option of invasive genetic testing for definitive karyotype determination. Loss rates following chorionic villus sampling (CVS) or amniocentesis for twins are not well studied, but the data that exist do not support markedly increased risks.

Comprehensive fetal anatomical surveys are recommended beginning at 16-18 weeks gestational age for MCMA twins. In addition to visualizing the fetal anatomy, amniotic fluid volume assessment should be performed, although it is less useful to help in the diagnosis of TTTS given the shared amniotic sac . Cord entanglement may be visualized but the there is no proven benefit to assessing severity of entanglement or trending this finding over time. The optimal frequency of interval sonographic testing for MCMA twins is controversial, with one reasonable strategy involving every-other-week assessments. More frequent assessments may be considered with more complicated presentations. Transvaginal assessment of the cervical length in the midtrimester also can provide insight into the risk of spontaneous preterm birth and allow further individualization of patient care.

Fetal echocardiography is also recommended at approximately 18-20 weeks due to increased rates of cardiac malformations in monochorionic twins and to acquired cardiac dysfunction observed in TTTS.

Given concern for a sudden vascular accident related to entanglement of twin umbilical cords, fetal heart rate testing is the recommended modality for antenatal assessment of MCMA twins. Although fetal demise may be a sudden and unprovoked event, abnormal findings that may herald a demise include the onset of repetitive variables for one or both twins that can occur in the absence of contractions. There is no single optimal time to initiate testing. Instead, the gestational age to start fetal heart testing should be determined on a case-by-case basis.

Surveillance should begin at a time when the patient would consider intervention if non-reassuring fetal heart testing indicating delivery were to be identified. In order to make this decision, the patient must be informed of the risks of an unmonitored fetal demise as well as the implications this may have on a co-twin (risk of co-death or survival with severe neurological injury) and have the opportunity to weigh this against the risks of iatrogenic prematurity for both twins related to indicated premature delivery. Perinatology and neonatology consultations may help to best inform the patient for this decision. In most cases, fetal heart testing is initiated between 24 and 28 weeks gestational age.

The optimal frequency of testing is controversial, with accepted practice patterns ranging from periodic testing to continuous fetal monitoring. With periodic fetal testing, there is a risk for a unmonitored fetal demise (or demises) between studies. However, even with a plan for continuous fetal monitoring, it is important to inform the patient that such monitoring is technically challenging and often imperfect. In one study, successful monitoring of both twins occurred roughly half of the time, with increased difficulty associated with earlier gestational age. Therefore, the patient undergoing testing for MCMA twins must be informed that even with an intention for continuous monitoring an unmonitored cord accident and related twin demise(s) can still occur. In addition, continuous fetal monitoring also presents potential maternal risks of physical deconditioning, psychologic deterioration and venous thromboembolic complications. Thus, at our institution, we generally perform twice-daily non-stress testing (NST) between 24 and 28 weeks, with continuous monitoring reserved for those women whose intermittent monitoring has raised concerns.

Although we usually administer corticosteroids for fetal benefit around the time that NST surveillance begins, there are no data to demonstrate benefit to the anticipatory administration of antenatal steroids for MCMA twins. Antenatal corticosteroids are recommended prior to the premature delivery of MCMA twins.

Given certain prematurity, MCMA twin delivery should be coordinated with neonatologists.

4. Complications

MCMA twins are at increased risk for congenital malformations. This risk is above that observed in MCDA twins, and may occur in as many as 25% of MCMA twins. Comprehensive fetal anatomical surveys as well as fetal echocardiograms are therefore essential components of antenatal care.

MCMA twins are at risk for all monochorionicity-specific complications seen in MCDA twins (see MCDA chapter). Of note, twin-twin transfusion syndrome (TTTS) is believed to be less common in MCMA twins for unclear reasons, with an incidence estimated to be roughly 5%. Given the obvious inability to demonstrate the polyhydramnios-oligohydramnios sequence in a monoamniotic pair, the diagnosis of TTTS can be more difficult to secure. In MCMA cases with TTTS, polyhydramnios is a requisite finding and Doppler and echocardiographic findings are important to support the diagnosis.

As noted above, the most serious complication facing MCMA twins is a life-threatening vascular accident related to entanglement of the twin umbilical cords. This complication can result in death of one or both twins. In earlier series, fetal mortality rates over 50% were reported. However, in more contemporary series with clinical practices including regular fetal surveillance and elective premature delivery, perinatal mortality rates of 10-20% are reported.

Following a monochorionic twin's death, its co-twin is at risk for profound injury around the time of the death due to hemodynamic instability within their shared placental circulation. The co-twin is at risk to transiently bleed into the reduced-pressure placental circulation and demised fetus, thereby hypoperfusing its brain and other vital organs. This places the co-twin at a roughly 10% risk of death (a double demise) or, if it should survive, a 15-20% risk of profound neurological injury. This insult is believed to occur instantaneously around the time of a twin death such that there is no evidence to support emergency cesarean delivery on behalf of the co-twin as a risk-reducing strategy.

Despite a lack of data to support this practice, some perinatologists recommend immediate delivery following a twin death after considering the gestational age. When expectant management is instead pursued, case reports exist of severe anemia detected within a surviving co-twin that has been successfully corrected with in utero transfusion. While there are limited data to conclusively demonstrate benefit with this practice, middle cerebral artery Doppler studies are recommended as part of the post-demise assessment of a surviving co-twin.

5. Prognosis and outcome

With contemporary practice, perinatal mortality of 10-20% can be expected in MCMA twin pregnancies, with higher rates observed when congenital malformations are identified. Among survivors, morbidity is generally not believed to be different from expected outcomes based upon gestational age at delivery. An exception to this is with survivors of a co-twin fetal demise, which places the survivor at substantial risk for profound neurological deficits.

6. What is the evidence for specific management and treatment recommendations